This invention relates to a metal cable or coaxial lead-in cable including at least one center conductor made of a metallic material, an internal insulator provided around the center conductor, a tape enclosure whose tape has a transverse width larger than the length of the outer circumference of the internal insulator and consists of an aluminum foil layer and a plastic layer laminated thereon, the tape being longitudinally wrapped closely around the internal insulator with the aluminum foil layer being inside the plastic layer, and an external insulating coating provided on the outer circumference of the tape enclosure.
Such a metal cable, and particularly a coaxial lead-in cable of the kind mentioned in the opening paragraph have been typically used for connecting terminals to a multiple digital network in telecommunication service. The metal cable in a communication line is required to have good communication characteristic and shielding property for avoiding the signal leakage and noise disturbance. In the multiple digital network, particularly, it is required for cables to have extremely high communication characteristic and shielding property. With the coaxial lead-in cables, more particularly, the good communication characteristic and shielding property are very important. Moreover, the coaxial cables to be used for connection of terminals are needed to have a construction capable of being freely bent, and therefore the external conductor must be flexible maintaining their shielding property.
In order to ensure the high shielding property and communication characteristic, aluminum pipes may be used as shielding members or external conductors, but they are disadvantageously poor in flexibility. In a hitherto used shielded metal cable or coaxial lead-in cable, therefore, as shown in FIG. 1a a tape 4 having a transverse width larger than the length of the outer circumference of an internal insulator 3 provided around a center conductor 2 and consisting of an aluminum foil layer 4a and a plastic layer 4b laminated thereon is longitudinally wrapped closely around the internal insulator 3 with the aluminum foil layer 4a being inside the plastic layer 4b. With this shielded metal cable, one edge portion 4A is overlapped on the other edge portion 4B, these edge portions 4A and 4B corresponding to the longer portions of the tape than the length of the outer circumference of the internal insulator 3, thereby forming a shielding member or an outer conductor around which is covered by an external insulating coating 5.
With the shielded metal cable or coaxial cable of the prior art as shown in FIG. 1a, however, this shielding member or external conductor as the tape enclosure is poor in high frequency shielding effect, because the aluminum foil layers at the overlapped tape edges do not contact with each other. Moreover, when this cable is subjected to bending or twisting, the bound portion of the tape tends to separate to lose the high frequency shielding effect with high probability. Such a reduction or loss in high frequency shielding effect will cause noise disturbance and signal leakage. With the digital signals different from the analog signals, slight noise disturbance and signal leakage will give rise to very serious damage to the associated telecommunication system.
Moreover, with another shielded metal cable or coaxial lead-in cable hitherto used, as shown in FIG. 1b, when a tape is wrapped closely around an internal insulator, aluminum foil portions of both the extra edges 6A and 6B not participating in wrapping are forced against each other to form a fin-shaped extra portion which is then folded upon the tape itself already closely contacting the internal insulator to form an overlapped portion thereon.
This coaxial lead-in cable of the prior art shown in FIG. 1b has been improved to keep the inner aluminum foil layers of the edge portions of the tape in contact with each other. However, as this contacting portion has no resistance to tensile forces generated when the cable is subjected to bending or twisting, there will be a risk of the contacting portion being separated.